The present invention relates to an electric arc furnace for melting metal, especially steel, which includes a container having an upper open end adapted to be closed by a cover, and which at least in the region of its lower hearth portion and in the lower wall portion, extending upwardly from the hearth portion beyond the highest level of a mass of molten material forming therein, an outer metal shell covered at the inner surface with refractory material and having substantially the form of a hollow cylinder and defining at least in the upper wall portion an essentially circular cylindrical inner furnace space for the reception of the material to be melted.
In an electric arc furnace of this kind the heat produced by the arc between a plurality of electrodes is used for melting of metal, which for instance in the form of scrap metal is filled by means of a charging basket into the interior of the furnace.
The hearth lined with refractory material receives the liquid steel forming during the melting of the scrap. The dimension of the hearth depends on the amount of liquid steel provided for one melt. Above the cylindrical portion of the hearth there is located a cylindrical wall portion to serve for the reception of a maximum amount of scrap. The furnace container usually has the same diameter in the cylindrical part of the hearth and in the upper cylindrical wall portion. The height of the container wall is determined by the weight of the scrap to be melted and the number of charges for one melt.
In the usual operating practice of electric arc furnaces, the furnace is filled by means of a charging basket several times with scrap. After the first charge, which fills the total volume of the furnace container, is molten therein, the furnace container is again charged and after melting of the second charge the third charging is carried out, and so on.
The number of charges depends on the bulk weight and the furnace volume. Each charging process results, in electric arc furnaces, in an interruption of the production and a high heat loss since the furnace cover with the electrode must be tilted away from the upper open end of the furnace container during the charging of the scrap thereinto.
The charging of the furnace for a melting operation in a plurality of successive filling operations is preferred over the charging with only one filling operation, especially for the reason that otherwise the furnace container for the reception of one and the same total charge would require considerably greater outer dimensions. Such a furnace container for a single charge for each melting operation would have the disadvantage of considerably higher capital outlays, which necessarily would disadvantageously influence the cost of the melting operation. The height of the furnace container is normally so dimensioned that for a single melt two to five filling operations are necessary depending on the quality of the scrap material to be fed into the furnace.
The refractory stones and refractory material which protect the walls of the furnace container are attacked by the electric arcs, as well as by the liquid steel and the slag. Therefore, the charging volume of the furnace container gradually increases during use thereof, due to the erosion of the refractory meterial. Therefore, after a predetermined time of use of the furnace the latter has to be newly lined, while at the end of this predetermined time at which the furnace container has to be newly lined, at least an additional charge has to be filled in the container to provide in the latter a bath of molten metal with the predetermined upper level.
In order to reduce the charging and leveling time during the first half of the useful life of the lining of the furnace container as compared to the second half of the useful life thereof, the lining of the furnace container in the upper region thereof was provided with shorter special refractory stones, the useful life thereof was at least equal to the normal longer stones.
A second step to enlarge the inner space of the furnace container was the use of water cooled wall elements. The up to now used lining of refractory stones over the whole height of the furnace container has thereby been replaced in the upper wall region, above the highest level of the molten metal bath forming therein, by water cooled wall elements which have been constructed especially small.